More Heat than Light?

Interest in the field of solar energy has centered until recently mainly on photovoltaic devices, which convert the sun's light into electricity. Now a team of researchers at a DOE Energy Frontier Research Center (EFRC) is opening an alternative path to transforming the sun's radiation directly into usable energy, with an important breakthrough in solar thermoelectric devices—which generate electricity from the sun's heat. The MIT-led team has developed a flat-panel solar thermoelectric cell that is comparable in efficiency to a photovoltaic cell—and seven to eight times more efficient than similar thermoelectric cells produced to date.

This advance begins to put solar thermoelectric technology on the map as a potentially plausible supplement to photovoltaics. The research also culminates a decade of important improvements in thermoelectric conversion efficiencies—improvements that could ultimately translate into a wide array of potential applications beyond solar energy, ranging from the capture and re-use of heat from vehicle exhaust to the recycling of massive waste heat from industrial and power plants.

"The development is potentially game-changing," said Gang Chen, Carl Richard Soderberg Professor of Power Engineering at MIT and Director of the Solid-State Solar-Thermal Energy Conversion Center, one of 46 EFRCs established in 2009 and supported by the DOE Office of Science.

The thermoelectric effect, on which this technology depends, is both simple and universal. If you heat one end of a wire while the other end remains cool, an electrical voltage arises across the temperature difference or gradient. Electrons migrate from the hot area to the cool area, and if connected, the wire could drive a circuit (in the case of a simple wire, of course, a very weak circuit). The process is also reversible. If you run an electric current through a junction made of two different wires joined into a loop, one end will cool and the other end will heat up. So, in essence, you can apply heat to generate electricity, or you can apply electricity to create a cooling effect. Heat gives you electricity; in reverse, electricity gives you cooling. Virtually all materials have thermoelectric properties, but only in certain materials is the effect significant enough to be of any use.

Thermoelectric devices are already deployed in the real world, mainly for specialized cooling and sensing applications; they are used to cool auto seats in the summer in some car models, for example. NASA has put them in spacecraft. Commercial thermoelectric devices are based on semiconductors—the most prominent being various alloys of a compound called bismuth telluride (Bi2Te3). But one reason that thermoelectric devices have remained basically a niche product is that these semiconductors, while having a very strong thermoelectric effect in relation to other materials, have not been terribly efficient either as cooling devices or as generators of electricity. Moreover, myriad efforts to improve their performance had shown very little progress for many years.

Enter nanoscience. In work during the decade prior to the establishment of the EFRC, Chen and colleagues began applying nanoscience to effect major improvements in the effectiveness of thermoelectric semiconductors.

One key to a strong thermoelectric effect is the ability to maintain a strong temperature differential or gradient across the device. It is the temperature difference or gradient between the hot and cool sides of the thermoelectric device that generates the electron flow. Similarly, the ability to use the device in cooling applications requires maintaining a differential between the cool and hot sides.

Consequently, the semiconductor material, which is sandwiched in between the hot and cool sides of the device, needs to be a poor conductor of heat. It should insulate rather than allow heat to pass through from the hot to the cool sides of the device, and thus help maintain the gradient.

Unfortunately, although compounds like bismuth telluride alloys are pretty good thermal insulators, in their bulk form they are not good enough for widespread application.

The ability of a material to conduct heat depends on something called phonons, which are a quantum-mechanical name for vibrations of atoms in the materials. The ease with which these phonons tend to spread through a material determines a material's heat conductivity.

In 1995, Chen's MIT colleague Mildred Dresselhaus (with her student Lyndon Hicks) published two landmark papers describing how fine layers or wires in thermoelectric semiconductors could significantly improve the performance of thermoelectric devices. The effect of the fine layers and wires, called quantum wells and quantum wires, is to create quantum mechanical effect on electrons. Another benefit of these quantum wells or quantum wires is their effect of disrupting the spread of phonons—imagine digging a series of long ditches in a field to slow the advance of an attacking army—and thus decreasing the conduction of heat.

This was a critical insight. Chen and Dresselhaus worked together for many years, beginning in the late 1990s, to understand the details of electron and phonon propagation in thermoelectric semiconductor materials, especially in alternating multilayer films called superlattices. However, the fabrication of superlattices is by nature a very painstaking and expensive process. Equipped with insights gained from studying these fine structures, Chen and colleagues searched for a more cost-effective method of achieving the same outcome and turned to nanoscience.

Chen and Dresselhaus enlisted Zhifeng Ren from Boston College, an expert in materials synthesis. By grinding alloys like those of bismuth telluride into tiny, nanoscale-size fragments and reassembling the fragments into solid semiconductors, the team managed to create multiple fissures within the material that similarly disrupted phonon spread, significantly reducing heat conductivity.

There is an engineering term called "figure of merit" that summarizes several parameters that together determine the effectiveness of a thermoelectric device. For nearly 50 years, the figure of merit for the alloy bismuth antimony telluride as a thermoelectric semiconductor had remained around 1—despite many efforts to improve its performance. Chen's team was able to fabricate a nanostructured semiconductor from this material and raise the peak figure of merit to 1.4—a 40 percent improvement. What's more, the researchers were able to achieve the result using a pair of inexpensive and common industrial techniques, known as ball milling and hot pressing. The work was published in the journal Science.

The next step was to incorporate the more efficient thermoelectric semiconductor into a device to convert solar energy into electricity. Here again, Chen and his team paid very close attention to concerns of cost-effectiveness in fabricating their device. At the heart of their device, of course, were the nanostructured semiconductors they had developed during the previous decade.

In work on solar thermoelectric solar cells, the conventional wisdom had been that sunlight should be concentrated using various optical components, focusing the light so that a temperature difference would be created between the hot and cold side, thereby generating electricity. However, optical focusing is expensive and requires costly tracking components. To reduce the cost, Chen and his team sought to concentrate heat rather than light, so that the final cell they developed is similar in structure to a normal, flat-panel photovoltaic cell.

To concentrate heat efficiently, they added two further innovations. Instead of using a simple black heat absorber to capture the sun's radiation, they used a high-performance wavelength-specific heat absorber for the hot side of the thermoelectric cell. The wavelength-specific design substantially increased heat absorption and reduced heat loss by re-emission. (For the cool side, the researchers used simple copper.)

In addition, to prevent heat loss from contact with the air, they borrowed an idea widely used in China, where millions of homes depend on solar hot water systems, which use direct sunlight to heat water for home use. The researchers enclosed the system inside a vacuum container. The cost-effectiveness of this approach is witnessed by the fact that more than 100 million square meters of these vacuum-type systems are deployed throughout China in household solar hot water systems, with typical lifetimes of up to 15 years.

As reported in the journal Nature Materials, using this design, Chen and his team were able to achieve a peak conversion efficiency of 4.6 percent under simulated sunlight conditions typical of temperate climates—an efficiency seven to eight times that of any non-optically concentrated solar thermoelectric cell so far developed.

The efficiency level is comparable to that of so-called Grade B solar photovoltaic cells—insufficient at this stage for a full-blown commercial system as a standalone unit, but more than adequate for a proof of concept. In the Nature Materials article, Chen and his team outlined several paths to improving efficiency, including further boosting the figure of merit for thermoelectric semiconductors, improving performance of various surfaces to minimize heat emittance, and using both optical and thermal means to better concentrate the sun's heat.

Thermoelectric devices are likely to play an expanding role in our energy economy in the coming years, not just in solar energy, but also in a wide range of applications where efficiencies can be gained through the capture and re-use of waste heat. The creation of the first solar thermoelectric cell to match photovoltaic levels of efficiency will likely be remembered as a major milestone in the development of this important technology.